Ok, I can't find it!
Unless you have a piece of carbon tube that is the perfect fit for the sleeve, then I'd not worry about that too much. What I have done previously is to laminate up a single layer sheet of glass weave (Peelply, Glass, Peelply on something flat). Make it large enough so that the width is around 1.5 times the circumference of the mast and perhaps half a metre in length. Sand up both sides of it so that it's keyed up nicely for bonding. Take both ends of your mast (the mast, and the new stump) and then sand up the inside of the tube for around 30cm each end and clean it up.
If you take the piece of glass and roll it up into a tube and then insert it into one end of the mast, when you let go of it, it'll obviously unroll itself to fit the internal diameter of the tube nicely. With luck, both of your tubes will have a similar ID, but the sprung tube method will hopefully take a bit of the slack out and fit snugly in both sides. Have a test fit and make the 'join' in the tubes nice. You can just but them up with a straight cut, or if you are feeling keen you can perhaps put a tapered cut in so that you don't have an obvious line. It'll probably spread the load better and be a better join (but I've done 3 with a straight butt joint and they're all still ok, so not essential).
Now is the key bit, you need to try and remove some carbon from the tubes that you are going to join together as you want to taper the outside of the tubes. This is essential for making a really nice and clean looking mast join. The aim is to replicate the mast laminate that already exists, but in the join. If you're feeling lazy, then you can just 'bandage' the carbon around the outside which will leave you with a 'lump' at the join, but as we all strive for composite perfection I know that you are going to taper the outside! I normally try and make my laminates cover around a 60cm area from memory. So you want to sand the carbon off from around 30cm from the join, so that you take enough carbon off so that when you replace it across the join, it is replacing what you are now sanding off. *See sexy diagram*. This sanding is easier to do now, before you have made your mast into one piece rather than 2.
Mix up a nice thick blend of epoxy and fillers. Coat up the inside of each of the pieces of tube with the bog so that when you insert the glass 'tube' it will spring out and stick to the inside wall. If you work out where the tube will overlap on itself, it's worth perhaps putting some bog on there as well so that the end is contained. Also, lastly, if you're being keen, you could align the overlap of the glass tube with the back of the mast as it might add a little stiffness (although this is unlikely to make much difference as that is what the carbon will do) You'll have a bit of time to work with this so spend time making sure it is straight. When I've done this, I've used some long lengths of wood and strapped them to the mast to make sure that it is in line, and not going to get knocked before the epoxy has gone off. Ok. Go and have a beer, you're done for the day!
Next session, you can remove all the wood and check that your join is good and the mast is straight. It's pretty solid at this stage and should be ok to move about without too much worry. Get your old friend the sandpaper out again and clean up the join ready for your next exciting laminating task! The key to the join, as mentioned earlier, is to place as much laminate into the join as is in the mast. I can't exactly tell you how much this will be, but from memory (which isn't quite what it used to be) I did a layer of weave, then maybe 5 layers of UD with a final layer of weave over the top. These layers need to be tapered across the join to spread the load, so these should match the tapering of the mast that you have removed by sanding. when I've done it before the first layer of weave was perhaps 10cm over the join, then steadily make the layers longer as you go over. So if the first one was 10cm, make the next layers of UD 20cm, 30cm, 40cm, 50cm and 60cm. Finally, you will want a nice layer of weave over the top. (You can decide whether or not you want to do this now, or if you want a clear laquer, it might be best to cure this lot and then do the final weave as a second process over the top). I normally wet out these cloths off the mast and really work the epoxy into them, and then transfer them onto the mast. I'd also work the cloths onto the tube with a foam roller to make sure that they are all flat and nicely bedded in. Peel ply is essential next so up to you how you do this. I like to cut some long strips, say 10cm wide, and then coil and wrap them around the mast from top to bottom. You can pull it nice and tight to get some good compression on the laminate BUT you need to be really careful that you aren't twisting the layup! Hold the laminate firm in one hand, and then carefully wrap and pull tight the peelply. I personally, then get the roller again and keep rolling the laminate till all of the peelply is thoroughly wet out with the excess coming through, you can do a secondary layer of peeply on top of this if you want (leave the ends of both of the wraps available and not stuck down so that you can pull the peelply off again!) I'd now re attach your wooden sticks just to ensure that it all goes off straight. Yay, it's beer time again.
Remove the peel ply and admire your handiwork. Sand it all back nice and smooth. If you didn't put the weave on in the first laminate, then you can do that now. It's only one layer, so it's easier to consolidate and you'll find it easier to get it really neat. Once that has been sanded then you can obviously lacquer or paint it.
Other notes... If this is up to or below the gooseneck, then you can't really make this bit of the mast too stiff. So if you don't think the 5 layers of UD is enough, then just do more.
